The obvious question is:
Is hreflang intended as a substitute for Content-Language
that doesn't require polling the server to determine the values
the server supports for that URI or as an override or
supplement to Accept-Language?
The only other attribute that exists that corresponds to
one of the Accept/Content pairs of HTTP is type, and
its description indicates that it corresponds to Accept,
not Content-Type.
Thus it would seem that if hreflang is restored to XHTML2
that either it should correspond to Accept-Language or
type should be changed to correspond to Content-Type.
(Question: would hrefcharset or hrefencoding be of any use?